Digital Logic TopicsBrief summary of characteristicsVisualization as a comparatorTruth TablesVoltage levelsSlide 6TTLCMOS Logic3/01/2003 BAE 5413 1Digital Logic TopicsBrief notes to preface the laboratory exercise3/01/2003 BAE 5413 2Brief summary of characteristics•Digital logic–Electronic elements for processing digital (numeric) information–Normally bi-state, 1 or 0 digital values represented by two discrete electrical signals–Our concern is primarily interfacing with logic inputs and outputs•Important terms/issues–Fan-out - the amount of output drive capacity compared to input drive requirement–Open collector - an output that is configured as a switch to reference voltage rather than a voltage level signal–Propagation delay - the amount of time between a change in input state and a change in output state.3/01/2003 BAE 5413 3Visualization as a comparator•Comparator•Vout = +5 if Vin 1 > Vin 2 else Vout = 0•Devices are available designed for the comparison purpose. They are in general faster and better suited for this purpose than a conventional op-amp.•Devices actually used in digital logic are much simpler internally than an Op Amp~Vout~+5+0Vin 1Vin 23/01/2003 BAE 5413 4Truth Tables•Consider the following and gate•A truth table “enumerates” the output state for every possible input state. See Example:ANDABC Input OutputA B C0 0 01 0 00 1 01 1 13/01/2003 BAE 5413 5Voltage levels•Common logic types (families)–TTL•Transistor-Transistor Logic - Classically has 0 and 5 V nominal logic levels. lower voltage logic is evolving•High power consumption, simple, low cost–CMOS•Low power consumption. (current flows primarily during transition and is low when at a particular state)•Static sensitive•More complex than TTL•Fabricated in wider voltage ranges (4000 series)3/01/2003 BAE 5413 6Voltage levels•Noise Margins• Comparison among logic families5VTTL5V CMOS3.3 LVTTL2.5V CMOS4000 CMOSVCC5 5 3.3 2.5 *10VOH2.4 4.44 2.4 2.3 9.95VIH2 3.5 2 1.7 7VT1.5 2.5 1.5 1.2 5VIL0.8 1.5 0.8 0.7 3VOL0.4 0.5 0.4 0.2 0.05GND 0 0 0 0 0*3-18VVcc +5VOHTypicalVOHMinimumVOLMaximumVILMaximumVIHMinimum3.4V2.7V0.8V2.0V0.5VGndNoise MarginInput Output3/01/2003 BAE 5413 7TTLAInputVccYOutputGNDVccGNDAInputYOutputTTL Open Collector Gate•Gate Electronic structureTTL Bipolar Gate3/01/2003 BAE 5413 8CMOS Logic•CMOS - Logic–Low power–3 - 18 V logic levels–Static Sensitive–Gate Electronic Structure•Example gate
View Full Document
Unlocking...